1. Field of the Invention
The invention is directed to orthodontic apparatus, and, more particularly, to ceramic orthodontic apparatus having base portions for directly attaching the apparatus to the surface of a tooth.
2. Background Information
Known orthodontic brackets typically comprise a base portion, which defines an archwire slot for receiving an archwire, and tie wings projecting outwardly from the base portion for attaching ligature wire to secure the archwire to the bracket. The base portion further defines a tooth abutting surface shaped generally to conform to the morphology of a tooth. The bracket is mounted to a tooth by applying cement to the tooth abutting surface and then positioning and pressing the bracket against the surface of the tooth. It is desirable to bond the bracket to the tooth with adequate strength to withstand the ordinary forces exerted by the musculature and orthodontic appliances connected to the bracket. However, if the bond strength between the bracket and the tooth is too strong it may be difficult to remove the bracket after treatment without damaging the enamel surface of the tooth.
Most known ceramic brackets have a smooth tooth abutting surface. Such brackets are generally mounted to a tooth by first applying a silane compound to the tooth abutting surface and then applying cement to the silane coated surface. The silane compound improves the strength of the bond between the bracket and the tooth. One problem associated with such brackets is that the debonding procedure is often time consuming and painful to the patient. Because of the strength of the silane bond and brittle nature of the ceramic bracket, such brackets often fracture upon removal leaving pieces of both ceramic and cement bonded to the tooth. These pieces must be removed with diamond burrs, which can be a time-consuming, painful procedure, that frequently damages the enamel surface of the tooth.
One type of known ceramic bracket has relatively deep rectangular, or square shaped pockets with sharp corners formed in the tooth abutting surface for receiving cement. The deep pockets are provided to increase the bonding surface area, and thus increase the strength of the bond between the bracket and the tooth. The shape and depth of the pockets, however, makes it difficult to intimately apply cement to the pocket surfaces. As a result, after the cement sets air gaps form between the cement and the surfaces of the pockets. Accordingly, the strength of the bond between the tooth and bracket decreases, and may cause the bond to fail under normal loading forces.
Another type of known ceramic bracket has one or more V-shaped grooves formed on the tooth abutting surface for receiving cement. The V-shaped groove is provided to improve the bond between the bracket and the tooth. However, one problem associated with both brackets having a V-shaped groove and brackets having rectangular or square shaped pockets, is that the sharp corners of the grooves or pockets are points of concentrated stress. This problem is especially critical in the region of the base of the archwire slot where the cross sectional area of the bracket is thinnest. The concentrated stress can cause the bracket to fracture between the bottom of the V-shaped groove or pocket and the base of the archwire slot when normally experienced forces are exerted on the bracket.
Another problem with known ceramic brackets occurs when the brackets are mounted to a tooth. When the brackets are pressed against a tooth excess cement is squeezed from underneath the bracket and against the base portion beneath the tie wings. This condition must be checked, and the cement must be removed several times while mounting the bracket to a tooth. If the cement is not removed, it will set in the space beneath the tie wing and prevent the ligature wire from fitting around the tie wing to secure the archwire to the bracket.
This problem has been solved with some metal brackets by extending a flange or base member outwardly from the tooth abutting surface of the base portion and beneath the tie wings. The flange prevents any cement from collecting beneath the tie wings and leaves sufficient space below the tie wings to wrap the ligature wire around the bracket. Ceramic brackets with V-shaped grooves or square pockets, however, cannot avoid this problem by extending a flange from the base portion beneath the tie wings. The flange must be thicker than at least the depth of the cement pockets or grooves in order for the bracket to have sufficient structural strength to withstand normal loading forces. Therefore, in order to provide a flange of sufficient thickness and maintain sufficient spacing between the flange and the tie wings for wrapping the ligature wire, the overall vertical height of the bracket must be increased. This condition is uncomfortable for patients. As a result, known ceramic brackets are generally not provided with a flange beneath the tie wings, and accordingly, such brackets require removal of excess cement from around the base portion of a bracket while mounting the bracket to a tooth.
Yet another disadvantage associated with known ceramic brackets is the problem of debonding the bracket from a tooth without fracturing the enamel surface of the tooth. Some known metal orthodontic brackets can be mounted to a tooth by applying adhesive to a screen mesh base. When debonding, the malleability of the mesh base permits the bracket, in effect, to be peeled from the surface of the tooth. As a result, such brackets are generally removable from a tooth without damaging the enamel surface of the tooth. Ceramic brackets, on the other hand, are rigid and brittle in comparison to the mesh base of a metal bracket. As a result, known ceramic brackets often fracture upon removal. If the strength of the adhesive bonds between the bracket and cement, and between the bracket and the surface of the tooth, are stronger than the enamel surface of the tooth itself, removal of the ceramic bracket may fracture the enamel surface of the tooth.
The orthodontic apparatus of the invention overcomes the problems and disadvantages of known orthodontic apparatus and provides an improved base portion for directly bonding the apparatus to the surface of a tooth.